1. Field of the Invention
The present invention relates to a pressure contact type semiconductor device and an electrode block usable in the same, and more particularly, it relates to an improvement for unifying a pressure distribution between an external electrode and a semiconductor substrate.
2. Description of the Prior Art
In a pressure contact type semiconductor device having a control electrode, an electrical contact of a semiconductor element with a control electrode conductor is kept by an elastic force of a spring, while external pressure is applied between an anode conductor and a cathode conductor provided in upper and lower portions of the semiconductor element.
FIG. 9 is a sectional view showing a conventional pressure contact type gate turn-off thyristor (GTO). The flat pack type GTO 200 includes a semiconductor substrate 2, and the semiconductor substrate 2, has a pnpn 4-layer structure (not shown). On the top major surface of the semiconductor substrate 2, a cathode electrode 10K and a gate electrode 10G made of aluminum are selectively formed. The cathode electrode 10K is positioned in the center portion of the upper major surface, while the gate electrode 10G is positioned around the same. On the bottom major surface of the semiconductor substrate 2, an anode electrode 10A made of molybdenum and functioning also as a temperature compensating plate is brazed. Thus, a semiconductor element 1 is composed of the substrate 2 and the electrodes 10K, 10G and 10A.
The anode electrode 10A is fitted into a guide ring 7, and this keeps the semiconductor element 1 in position. In lower portion of the anode electrode 10A an external anode electrode 3 made of copper is disposed, while in an outer circumference of the external anode electrode 3 a flange 8b made of covar is brazed. Onto a lower lip of an insulating cylinder 5 made of ceramic another flange 8a is attached. The flanges 8a and 8b are brazed to each other to connect the insulating cylinder 5 with the external anode electrode 3. The insulating cylinder 5 is fitted on a guide ring 7.
On the other hand, in upper portion of the semiconductor element 1, an external cathode electrode 20 made of copper is disposed. The external cathode electrode 20 has a convex portion 21 in the center part of its bottom surface, and the convex portion 21 is united with a circumferential portion 22. In an area of the circumferential portion 22 adjacent to the convex portion 21, an annular groove (concave portion) 25 is formed. The convex portion 21 is in electrical contact with the cathode electrode 10K through the temperature compensating plate 11. The groove 25 is so positioned as to face the gate electrode 10G.
On the gate electrode 10G, an annular gate conductor 13 made of metal is disposed. The gate conductor 13 is electrically isolated from the external cathode electrode 20 by an insulating sheet 12.
As shown in a partial enlarged view of FIG. 10, an insulating ring 44 and a flat washer 43 are disposed on the upper surface of the gate conductor 13. Another flat washer 41 is placed on the ceiling face of the groove 25, and a pair of conical springs 42 intervening between the flat washers 41 and 43 permit the gate conductor 13 to be in pressure contact with the gate electrode 10G (FIG. 9).
Referring back to FIG. 9, one end of a gate lead 6a made of silver is brazed to the gate conductor 13. The gate lead 6a is held in an insulating sleeve 6b and extends outwords from an insulating cylinder 5 through a bore formed in the wall of the insulating cylinder 5. The gate lead 6a is inserted into a tube 6c made of metal and welded to the tube 6c at its tip 6d. In this way, a gate electrode 6 is formed.
A flange 4 is attached to an outer circumference of the external cathode electrode 20, and the flange 4 is brazed to an upper lip of the insulating cylinder 5 to connect the external cathode electrode 20 with the insulating cylinder 5.
When the GTO 200 having the structure as stated above is employed in an electric equipment, the GTO 200 should be positioned between an anode member 51 and a cathode member 52 of the equipment. The anode and cathode members 51 and 52 are pressed in a (-Z) direction and a (+Z) direction by springs, respectively, so that the bottom surface of the anode member 51 is in press contact with an upper surface 23 of the external cathode electrode 20 while the upper surface of the anode member 52 is in press contact with a bottom surface 3a of the external anode electrode 3. The pressure force keeps the external cathode electrode 20 in electrical contact with the cathode electrode 10K through the temperature compensating board 11 and also keeps the external anode electrode 3 in electrical contact with the anode electrode 10A. Under such conditions, voltage is applied between the cathode member 51 and the anode member 52, and a gate signal is applied to the external gate electrode 6 so as to turn on or off the GTO 200.
The external cathode electrode 20 and the external anode electrode 3 are so called stamp electrodes or post electrodes, and the surfaces 23 and 3a are post electrode faces.
In the above-mentioned GTO 200, to attain low values of a contact resistance between the cathode electrode 10K and the external cathode electrode 20 through the temperature compensating plate 11 and of a contact resistance between the anode electrode 10A and the external anode electrode 3, it is necessary to apply external force of approximately several tens kg/cm.sup.2 to several hundreds kg/cm.sup.2 uniformly to both the post electrode faces 23 and 3a. For the purpose, a pressure applied from the cathode member 51 and the anode member 52 to both the post electrode faces 23 and 3a should be considerably large.
The external force partly applied to the convex portion 21 and the external cathode electrode 20 acts between the external cathode electrode 20 and the cathode electrode 10K. However, the external force applied to a circumferential portion 22 of the external cathode electrode 20 doesn't act between the electrodes 20 and 10K but acts as a bending moment of the external cathode electrode 20 around a point S shown in FIG. 10. This is because there is no member for supporting the circumferential portion 22 from below, but merely an elastic force of the conical spring 42 acts upwards.
Thus, as schematically shown in FIG. 11, the circumferential portion 22 of the external cathode electrode 20 is bent by an external force P applied from the cathode member 51 of the external equipment to the post electrode face 23. Accordingly, the shape of the groove 25 is deformed, and the pressure applied from the conical spring 42 (FIG. 10) held in the groove 25 to the gate conductor 13 is also changed. As a result, the pressure propagated from the gate conductor 13 to the semiconductor substrate 2 through the gate electrode 10G is changed, and there arises the disadvantage that a gate characteristic in the GTO 200 becomes unstable.
Additionally, the bending moment around the point S exerts effects on a stress distribution in the convex portion 21, and therefore, a spatial distribution of the pressure applied from the convex portion 21 to the cathode electrode 10K becomes uneven. Then, a contact resistance between the convex portion 21 and the cathode electrode 10K with the temperature compensating plate 11 intervening therebetween become spatially uneven, so that current is concentrated in an area having a small contact resistance. This causes the semiconductor element 1 to locally generate larger heat while the GTO 200 is turning on, and there arises a problem that a thermal breakdown of the semiconductor element 1 is easily caused.